Increasingly stringent regulations governing the emissions of power plants will force power plant operators to run selective catalytic reduction (SCR) systems year round in order to reduce nitrous oxide (NOx) emissions. Currently, most power plants utilize their SCR systems only during an “ozone season”, a period from May to September when ozone emission must be controlled especially carefully.
The ozone season corresponds to a period of peak electrical demand when power plants are running at maximum capacity. Therefore, existing SCR systems were designed to be operated within a narrow range of exhaust temperatures corresponding to the exhaust temperatures reached by power plants operating at that maximum capacity, also known as maximum continuous rating (MCR). For example, SCR systems may have a maximum operating temperature of about 700° F. at full load and a minimum operating temperature for catalyst operation of about 620° F. This difference between maximum and minimum SCR operating temperatures defines the SCR control range of the power plant. At low load the flue gas temperature produced by the power plan may be only 580° F., well outside the SCR control range.
When power plants are operated at less than their MCR, (e.g., at low load), their exhaust temperatures are reduced accordingly. Many power plants operate at less than MCR for six or seven months of the year. This presents a problem in that, for most of the year, power plants do not produce exhaust gases within the relatively narrow temperature range required by their existing SCR systems.
One approach to complying with the more stringent ozone regulations would be to replace the existing SCR systems with new systems designed to operate at a wider range of temperatures corresponding to various power plant output levels. However, installing the new systems would represent a substantial financial investment, the new systems would be significantly larger than the existing systems (up to an order of magnitude larger) and would require extensive, often infeasible, retrofitting design modifications.
In order to avoid having to install new SCR systems, various methods have been proposed to keep the exhaust temperature within the range of the existing SCR systems even when the power plant operates at reduced loads. These methods include economizer resurfacing, gas bypass systems, and split economizers, all of which present their own substantial design and cost limitations.
The increasingly stringent regulations continue to place pressures upon electric utilities to reduce plant emissions. Replacing the existing SCR systems, which have limited operating conditions, is not an economic possibility at most power plants. In addition, the above-described modifications to existing power plants are often problematic due to their space requirements and their high maintenance and installation costs. Therefore, improvements that allow for more economic and space efficient modifications to existing power plants are required.